1. Field of the Invention
The present invention relates to a surface acoustic wave element, surface acoustic wave device, and a communication device including the same. The surface acoustic wave element according to the present invention includes surface acoustic wave filter elements, surface acoustic wave resonators and the like. The surface acoustic wave device according to the present invention is used for mobile communication devices such as portable phones.
2. Description of the Related Art
Surface acoustic wave filters have been widely used as bandpass filters used for the RF (Radio Frequency; wireless frequency) stages of mobile communication devices including portable phones, automobile telephones and the like. In addition to miniaturization, adapting to wider passband and higher frequency supporting communications carriers, improvement of out-of-passband attenuation and insertion loss within the passband are required for these surface acoustic wave filters.
Recently, reduction of the number of components has been accelerated to realize miniaturized, light and low cost mobile communication devices and the like, and therefore, surface acoustic wave filters are required to have additional functions.
One of such functions is an unbalanced-balanced conversion function which allows the surface acoustic wave filter to be constructed as unbalanced input-balanced output type or balanced input-unbalanced output type.
Here, balanced input or balanced output refers to input or output of signal as an electrical potential between two signal lines, where the amplitudes are equal and the phases are opposite between the signals of the respective signal lines. On the other hand, unbalanced input or unbalanced output refers to input or output of signal as an electrical potential of one signal line with respect to the ground potential.
A major part of conventional surface acoustic wave filters are unbalanced input-unbalanced output type surface acoustic wave filters (hereinafter, simply referred to as “unbalanced type surface acoustic wave filter”). For this reason, when circuits and electronic components connected in stages subsequent to an unbalanced type surface acoustic wave filter are of balanced input type, a unbalanced-balanced converter (hereinafter, this converter is simply referred to as “balun”) is required between the unbalanced type surface acoustic wave filter and the subsequent stages. Also, when circuits and electronic components in stages prior to an unbalanced type surface acoustic wave filter are of balanced output type, a balun is required between the prior stages and the unbalanced type surface acoustic wave filter.
Accordingly, in order to provide an unbalanced type surface acoustic wave filter itself with the function of a balun, development of unbalanced input-balanced output type surface acoustic wave filters produced by providing the unbalanced-balanced conversion function to this unbalanced type surface acoustic wave filter, and balanced input-unbalanced output type surface acoustic wave filters produced by providing the balanced-unbalanced conversion function has been proceeding for practical use. In addition, improving the balance degree of balanced-type surface acoustic wave filters has been proposed.
For example, in the case of a surface acoustic wave filter 900 with a multielectrode configuration shown in FIG. 32 includes three IDT (Inter Digital Transducer) electrodes 901, 902, 903 arranged along the propagation direction of surface acoustic waves. In addition, it includes reflector electrodes 904, 905 disposed to sandwich the foregoing three IDT electrodes 901, 902, 903 (Refer to Japanese Unexamined Patent Publication No. 6-204781 (1994)).
The IDT electrode 902 excites surface acoustic waves by application of an electric field from the unbalanced input terminal 906 to one of the couple of comb-shaped electrodes disposed to oppose to each other. The excited surface acoustic waves are propagated to the IDT electrodes 901, 903 located on both sides of the IDT electrode 902. Then, a signal is output from an output signal terminal 907 that is connected to one of the comb-shaped electrodes of the IDT electrode 901, and a signal is output from an output signal terminal 908 that is connected to one of the comb-shaped electrodes of the IDT electrode 903, by which the surface acoustic wave filter 900 can output balanced signals.
This structure provides the surface acoustic wave filter 900 with an unbalanced input-balanced output conversion function as a balun.
A surface acoustic wave filter 910 shown in FIG. 33 includes three IDT electrodes 911, 912, 913 arranged adjacent to each other along the propagation direction of surface acoustic waves, and reflector electrodes 914, 915 disposed to sandwich these IDT electrodes (Refer to Japanese Unexamined Patent Publication No. 11-97966 (1999)).
The IDT electrode 912 is formed by a couple of comb-shaped electrodes one of which is separated into two parts. The separated two parts of the comb-shaped electrode are connected to balanced signal terminals 916 and 917, respectively, so that they are cascade-connected acoustically and series-connected electrically to each other.
The IDT electrodes 911, 913 are each shaped so that they have the reverse polarity with respect to the IDT electrode 912, and connected to one unbalanced signal terminal 918.
This structure allows the surface acoustic wave filter 910 to have an unbalanced-balanced conversion function as a balun. In addition, the impedance (about 200Ω) on the side of the balanced signal terminals 916, 917 can be about four times the impedance (about 50Ω) on the side of the unbalanced signal terminal 918.
Japanese Unexamined Patent Publication No. 2002-84164 discloses a cascade-connected resonator type surface acoustic wave filter with a structure for improving the balance degree, which includes three IDT electrodes arranged in the surface acoustic wave propagation direction, in which the IDT electrode located at the center comprises an even number of coupled parts.
A surface acoustic wave filter 920 shown in FIG. 34 comprises an electrode pattern 941 of a surface acoustic wave element that includes three IDT electrodes 921, 922, 923 arranged adjacent to each other along the surface acoustic wave propagation direction, and reflector electrodes 924, 925 disposed to sandwich these IDT electrodes.
The IDT electrode 922 is formed by a couple of comb-shaped electrodes one of which is separated into two parts. The separated two parts of the comb-shaped electrode are each connected to balanced signal terminals 926, 927 so that they are cascade-connected acoustically and series-connected electrically.
The IDT electrodes 921, 923 are each shaped so that they have the opposite polarity with respect to the IDT electrode 922, and both of the IDT electrodes 921, 923 are connected to an unbalanced signal terminal 928.
A reactance (capacitance) component 929 is connected to the line connecting the IDT electrode 922 to the balanced signal terminal 926.
Furthermore, a surface acoustic wave filter 930 shown in FIG. 35 comprises an electrode pattern 942 including IDT electrodes 931, 932, 933 arranged adjacent to each other that are interposed between the IDT electrodes 921, 923 and the unbalanced signal terminal 928 of the foregoing surface acoustic wave filter 920 of surface acoustic wave element, and reflector electrodes 934, 935 disposed to sandwich these IDT electrodes. This electrode pattern 942 of a surface acoustic wave element and the electrode pattern 941 of a surface acoustic wave element are cascade-connected (Refer to Japanese Unexamined Patent Publication No. 2004-96244).
The IDT electrode 932 is connected to the unbalanced signal terminal 928, and the IDT electrode 931 is connected to the IDT electrode 921 through a connecting line and the IDT electrode 933 is connected to the IDT electrode 923 through a connecting line.
In this manner, the surface acoustic wave filter 930 is capable of further increasing the attenuation out of the passband as compared with the surface acoustic wave filter 920.
Besides the foregoing examples, there is another known example as in Japanese Unexamined Patent Publication No. 2003-46369, in which IDT electrodes are formed so that they are asymmetrical with respect to a virtual central axis that is perpendicular to the propagation direction of surface acoustic waves.
While the foregoing surface acoustic wave filters have been proposed so far, particularly in recent years, improvement of the amplitude balance degree and phase balance degree within the passband has been required. These amplitude balance degree and phase balance degree are measured as difference in electrical potential between two input or output signal lines. Here, “amplitude balance degree” refers to a value indicating the degree of equality between the amplitudes of signals flowing through the respective signal lines, and as the amplitudes become equal, unwanted signals are canceled because of the amplitudes approximated to each other so that the value becomes close to 0 dB, resulting in an improved amplitude balance degree. In addition, the “phase balance degree” refers to a value indicating the degree of phase inversion between two input or output signal lines, in which the closer to 180° the phase difference is, the closer to 0° is the phase balance degree, that is, the phase balance degree is improved.
Meanwhile, the surface acoustic wave element (surface acoustic wave filter) 900 (see FIG. 32) disclosed in Japanese Unexamined Patent Publication No. 6-204781 is arranged so that the phases of signals output from the IDT electrodes 901, 903 are reverse to each other. In addition, the balance at the surface acoustic wave element 900 is controllable by changing the pitch of the electrode fingers of the IDT electrodes 901-903, or changing the distance between these IDT electrodes.
However, such a structure has a drawback that the balance degree is prone to deteriorate.
In the case of the surface acoustic wave element (surface acoustic wave filter) 910 (see FIG. 33) disclosed in Japanese Unexamined Patent Publication No. 11-97966, the electrical polarity between the adjacent electrode fingers of the IDT 911 and the IDT 912 is different from that of the adjacent electrode fingers between the IDT 912 and the IDT 913. Accordingly, the parasite capacitances generated at the balanced signal terminals 916, 917 are different from each other. For this reason, the balance degree of the surface acoustic wave element 910 is degraded.
Furthermore, in the case of the surface acoustic wave element disclosed in Japanese Unexamined Patent Publication No. 2002-84164, when a LiTaO3 single crystal substrate is used as the piezoelectric substrate, only an amplitude balance degree of about 1.2 dB and a phase balance degree of about 11 deg. can be achieved. As a result, this surface acoustic wave element fails to achieve a sufficient balance degree.
Meanwhile, the effect was examined on the surface acoustic wave element (surface acoustic wave filter) 930 (see FIG. 35) disclosed in Japanese Unexamined Patent Publication No. 2004-96244.
FIG. 36 is a graph showing the relationships between frequency and insertion loss of the surface acoustic wave element 930 and a surface acoustic wave element 930′.
Here, the device used as the surface acoustic wave element 930′ used as comparative example was one as shown in FIG. 37 that lacked the reactance (capacitance) component 929 in the surface acoustic wave element 930.
In FIG. 36, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents insertion loss (unit: dB). The broken line in the graph shows an insertion loss characteristic curve of the surface acoustic wave element 930 in FIG. 35, and the solid line in the graph shows an insertion loss characteristic curve of the surface acoustic wave element 930′ shown in FIG. 37.
According to the characteristic curve (broken line) of the surface acoustic wave element 930, the ripple within the passband increases as compared with the surface acoustic wave element 930′. This is believed due to a change in transmission characteristic caused by the structure of the surface acoustic wave element 930 in which a capacitance component is parallel-connected to the balanced signal terminal 926 as the reactance component 929, by which a capacitance component is additionally parallel-connected to one of the balanced signals.
FIG. 38 is a graph showing the relationships between frequency and VSWR (Voltage Standing Wave Ratio) of the surface acoustic wave element 930 and surface acoustic wave element 930′.
In FIG. 38, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents VSWR. The broken line in the graph shows a characteristic curve of the surface acoustic wave element 930 shown in FIG. 35, and the solid line in the graph shows a characteristic curve of the surface acoustic wave element 930′ shown in FIG. 37.
According to the characteristic curve (broken line) of the surface acoustic wave element 930, VSWR is deteriorated as compared with that of the surface acoustic wave element 930′. This is believed due to a change in reflection characteristic of the surface acoustic wave caused by the addition of the capacitance component (reactance component 929) parallel-connected to one of the balanced signals.
FIG. 39(a) is a graph showing amplitude balance degrees in the vicinity of the passband of the surface acoustic wave element 930 and surface acoustic wave element 930′, and FIG. 39(b) is a graph showing phase balance degree in the vicinity of the passband of the surface acoustic wave element 930 and surface acoustic wave element 930′.
In FIG. 39(a), the horizontal axis represents frequency (unit: MHz), and the vertical axis represents amplitude balance degree (unit: dB). In FIG. 39(b), the horizontal axis represents frequency (unit: MHz), and the vertical axis represents phase balance degree (unit: deg.). In addition, the broken lines in the graphs show characteristic curves of the surface acoustic wave element 930 shown in FIG. 35 (the upper and lower broken lines reflect two different levels of reactance component 929), the solid lines in the graphs show the results of measurements on the surface acoustic wave element 930′ shown in FIG. 37.
According to FIG. 39(a), the surface acoustic wave element 930 has values more distant from 0 dB than those of surface acoustic wave element 930′, i.e., the amplitude balance degrees are not good. This is believed because even when a capacitance component (reactance component 929) is parallel-connected and added to the balanced signal in the surface acoustic wave element 930, the difference in amplitude cannot be compensated.
In addition, according to FIG. 39(b), the surface acoustic wave element 930 has values more distant from 0 deg. than those of surface acoustic wave element 930′, i.e., the phase balance degrees are not good. This is believed because even when a capacitance component is parallel-connected to the balanced signal in the surface acoustic wave element 930, the difference in phase cannot be compensated.
As shown in FIGS. 36, 38 and 39, no significant improvements are observed in terms of insertion loss, VSWR, amplitude balance degree, and phase balance degree about the surface acoustic wave element 930 despite the addition of a capacitance component as reactance component 929 parallel-connected to one balanced signal terminal 926, and some characteristics are even degraded.